This invention relates to a fuel supply system for gaseous fuel operated vehicle engines, which is adapted to supply a compressed natural gas (CNG) to an engine after decompressing and mixing with air, and to a regulator suitable for use in the fuel supply system.
Recently, natural gases are re-evaluated as a substitute energy source for petroleum, and attempts are being made to use natural gases as a fuel for motor vehicles. The primary difference of the natural gase from petroleum resides in that the natural gas contains methane of low boiling point as a major component in contrast to the petroleum gas which is mainly constituted by propane and butane of relatively high boiling points. Accordingly a difficulty is encountered when using the natural gas as a fuel of motor vehicles because it is difficult to store it in liquefied form at normal temperature in the fashion of the liquefied petroleum gas (LPG) which is used as a fuel on certain kinds of motor vehicles. It is therefore the usual practice to store the natural gas in the form of compressed natural gas (CNG).
On a motor vehicle which uses CNG as a fuel, CNG which is normally contained in a bomb under high pressure, for example, under pressure of 200 kg/cm.sup.2 is decompressed, for example, to the atmospheric pressure by a regulator and supplied to an engine after mixing it with air by a mixer which is constituted by known component parts including a venturi. Citing an example of actually commercialized cars, the CNG bomb is mounted in a luggage room section, namely, in the trunk room in case of a passenger car and on the loading platform in case of a truck, and the fuel gas is supplied from the bomb under high pressure to a regulator which is located in the engine room.
As the regulator is required to decompress the fuel gas to a great degree as mentioned hereinbefore, it is usually constituted by two or a larger number of integral or separate decompression stages. Anyway, the regulator is mounted in an engine room section of a vehicle, necessitating to extend a high pressure gas conduit to the engine room section from the bomb in the trunk across a passenger's room section. Needless to say, greater the length of the high pressure conduit, higher becomes the possibility of gas leakage from joint or other portions of the pipe.
There has also been known a regulator of the type which employs a diaphragm in association with a gas flow control valve, the diaphragm being responsive to the air pressure in a decompressing chamber to control the gas flows into the decompressing chamber from a high pressure gas passage led from a fuel gas bomb. This arrangement is used in two or a greater number of separate or integrally combined decompressing stages to depressurise the CNG from the bomb, for example, from 200 kg/cm.sup.2 to the atmospheric pressure (Japanese Laid-open Patent Application No. 59-165852).
However, the pressure reduction by such an arrangement involves a problem that the terminal open end region of the high pressure gas conduit is cooled by adiabatic expansion of the fuel gas as it is released into the decompressing chamber from the high pressure conduit, freezing propane, butane, water or other gas components of relatively high melting point in that region, especially in the high pressure gas conduit, as a result narrowing the effective area of the conduit to block the fuel gas flow and lowering the performance quality of the engine.
In order to prevent water vapor in the gas from freezing due to adiabatic expansion of the gas during the sudden decompression, there has been proposed a regulator which is arranged to heat the circumference of the high pressure gas conduit by the use of cooling water which has been heated by the engine operation. However, in a case where the regulator is positioned in the vicinity of a fuel gas bomb in a luggage room section in the rear portion of a vehicle for reducing the length of the high pressure gas conduit to a minimal, the heating effect is lowered by heat dissipation from the cooling water conduit which has to be extended over a long distance between the engine room in the front portion of the vehicle and the luggage room in the rear portion.